This spline measures a third the
thickness of the pieces to be joined,
with the grain running perpendicular
to the spline length.

Round over the spline corners for
a tight fit in the slot, but cut the
splines 1/32" narrower than the
combined depths of the slots.

A push pad holds this test scrap
firmly against the router fence. Add
a high auxiliary fence to keep long
workpieces from tilting.

Splines provide inner strength

Splines create a face-to-face glue surface that resists flexing. Use through splines for an easy-to-make connection with visible splines. Mark each joint on its top face and set the saw blade 3/4" high? -- half the length of the splines. Orient the top faces of each piece against your rip fence for consistent groove positions between pieces. To keep extra-long pieces steady, add an auxiliary fence. Using a backer block to stop tear-out, cut kerfs on the ends to form a groove as wide as one-third the thickness of your stock.

Next, plane and saw the spline blank stock to match the width and combined depth of the grooves. You can make splines from plywood or solid stock. If you choose solid stock, as shown right, orient the spline grain with the workpiece grain. Insert the spline; then glue and clamp the pieces.

For not much more work, create a concealed spline, like the one shown in middle photo, that disappears after you assemble the joint. We made this joint using a 1/4" straight bit on a table-mounted router. Set the bit height to just more than half the width of your splines. Then adjust your router table fence to center the bit on the thickness of the workpiece ends.

Next, build a simple jig to guide your workpieces. From scrap slightly thicker than your workpieces, cut two stopblocks. Space them a distance apart that's twice the width of your workpiece minus mortise insets from both edges. Then attach a crosspiece that's 1/2" wider than the bit height for added safety and control. Clamp the connected stopblocks of the finished jig to the router table fence so they're equal distances from the bit, as shown bottom right.

To keep minor fence adjustment errors from creating an uneven joint, mark the top faces of your workpieces and have them facing you while routing the slots. Using a pushpad, press the workpiece against the router fence and down the edge of the right stop block to the router table. Slide the workpiece to the left stop block, as shown bottom right, and raise it clear of the bit.

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